Effects of Time Travel Trivia Quiz

Answer: Major changes in the past have major effects on the present

McCoy, out of his mind after a medical accident, travels back in time in "City on the Edge of Forever". He saves the life of Edith Keeler, changing history to the point where the Enterprise never existed. Kirk and Spock have to travel back and prevent him making that change. Given a normal human belief in cause and effect, it seems sensible that changing some event in the past will affect the present, and that changing major events will have major effects.

However, like a lot of time-travel plots, this one conveniently allows Kirk, Spock and the rest of the landing party to still exist and still remember the "original" past and present, so they know what has changed and what they need to change back.

Answer: Certain events are fixed and "have to happen", other changes are allowed

In both "The Fire of Pompeii" and "The Waters of Mars" the Doctor protests that some things are too important to history and must not be changed. But most of the time, he's happy to interfere in any situation - after all, once you've been to the end of time, everything is history.
Any series that deals with time-travel so frequently is bound to have some inconsistencies in how this is handled - sometimes the Doctor's behaviour in the past has effects on the present, such as the founding of Torchwood, yet in "The Unicorn and The Wasp" the Doctor helps history turn out the way we know it - by causing Agatha Christie's "missing day".
The concept of "certain things have to happen" comes up frequently, mainly because without those pivotal events it seems the present would have to be very different. What is pivotal though is always an interesting debate.

Answer: Changes in the past directly affect the present day

In line with "cause and effect", this movie shows Sullivan causing changes in the past and those changes have a clear and sensible effect on the present.
The first change John makes is to tell his father, a firefighter, how to escape the fire that originally killed him. However, this change also allows a serial killer to keep on killing and the two Sullivans have to work out what else changed and work together across time to catch the killer.
There are some really cool moments when we see the past changing the present, right up to the fascinating final fight.

Answer: Sam changed things to the "history" we know

While being unable to stop Oswald from firing, Sam becomes the secret service man (Clint Hill) seen leaping onto the back of the presidential vehicle and everything occurs according to "our" history. But Al tells us that in their "original" history, Jackie Kennedy had also died. In most of the episodes, Sam's changes seem to only make things better for the people he was directly involved in, and there are only a few cases where we see his actions having any effect on the present Al is living in.

But it is suggested that those changes also cause wider-ranging changes as each person's changed life also affects those lives around them.

Answer: Marty could change the future, but not what he'd done on his first time-trip

In the first "Back to the Future", Marty directly changes the past, but has a short time in which to change things back so he doesn't fade from history. In the second film, Marty has to undo the changes that "old" Biff (from the future) has made, without interfering with his "first-time" self as coming face-to-face with himself could cause a catastrophe.

This film is fascinating in showing the viewer some of the same events as the first film, but from a different perspective, with "second" Marty weaving through the gaps left by his original self. Again, the protagonist has at least a little while in which he remembers the original "present" and has a chance to change things back.

However, as Marty is the one who has caused the changes, this seems more acceptable as otherwise there would be the paradox of Marty never having existed, so he couldn't have caused the changes, so he would've existed...

Answer: There are minor changes, restricted to Eureka itself

The only changes are that several of the characters have changed jobs or relationships. Henry explains this as a "ripple effect" - only Eureka itself has any changes because that was the only place that was affected, the changes die out as you move further from the change point. This is a fairly common viewpoint, if the only things that changed were "trivial", maybe the changes that result from them would not be very large either.

It is another nice theory for TV/movie producers, they can show us changes in their own little setting, but don't need to worry about changing the rest of the world.

However, the changes themselves here make no sense - why would a few stray people running around in the 1940s lead Alison into a whole different career path (presumably chosen and studied long before she reached Eureka).

Answer: Nothing changes, the past happens exactly as it was recorded

One time travel theory says that what has already happened is what has already happened, including the presence of time travellers and any actions they may have taken. So the travellers were "always" fated to go back and perform those actions. This theory takes care of any paradoxes, instead of the past actions preventing the time travel, they are part of what dictates it. In the episode, Rebecca and Jack have no memory of what happened, but the sequence of events Pete and Myka go through in chasing a killer with an artifact are an exact match to what Rebecca and Jack supposedly did.

But that doesn't mean the killer was who they thought it was.

Answer: Memories

In "Groundhog Day" and "12:01" the protagonist immediately has full memories of the day they are reliving. In "Stargate: SG-1" Jack & Teal'c start by just thinking the day seems familiar, but as they go around and around the timeloop, their memory and knowledge becomes clearer.
This suggests that time-travel can cause major changes - as far as everyone else is concerned the whole day continues in response to what happens earlier as if that is the way things have always been. But again, for the sake of story-telling at least one person has to somehow be aware that time is looping.
The "Star Trek:The Next Generation" episode "Cause and Effect" takes this further, with the crew having to work out they are in a time-loop, decide what they need to know and find a way to send a message to themselves in the next loop.
"Window of Opportunity" also provides a fun viewpoint on what you might do if you knew you had infinite days to get your job done.

Answer: Having accidentally changed history, the crew struggle to put things back on track

In the episode "Tikka to Ride", the crew travels back in time in search of a good curry. Having accidentally stopped Lee Harvey Oswald from shooting the president, they then have to try and re-instate history so they can get back to their own timeline.
Again, the theory is that changing a major event has a drastic effect on the future.
While not one of their better episodes, the final payoff in terms of "Who shot JFK?" is absolutely classic. As Lister says, "It'll drive the conspiracy nuts crazy".

Answer: The past can be changed, but the universe dictates certain things will still occur

Nero, a tormented ship's captain, travels back in time and causes major changes, including the death of George Kirk (James T's father) and the destruction of the planet Vulcan. Nonetheless the all-important crew of the Enterprise are still brought together to take their historic place in the universe.
Given the major changes, some of the convolutions needed to gather our heroes seem very far-fetched. However, this "certain things must happen" is always a nice storyline, reassuring us that soulmates will always be together and our favorite heroes will always gather when we need them.